1. Field of the Invention
The present invention generally relates to automated inspection systems for identifying geometric characteristics of physical objects by transmission and reception of reflected sonic waves and, more particularly, to ways and means for compensating for atmospheric variables that affect operation of such systems.
2. State of the Art
In manufacturing assembly operations, including operations performed by robotic assembler machines, it is usually important that components are non-defective and are presented for assemblage in particular spatial orientations. These requirements arise because conventional assembler machines lack sufficient dexterity and machine intelligence to distinguish non-conforming parts from acceptable parts, or to handle objects that are not presented in certain prescribed positions. As a result, automated assembly systems often include custom-designed inspection and parts feeding systems whose cost may be several times the cost of the assembler machines. Also, inspection and feed systems often must be redesigned for new components that are to be automatically assembled. Thus, while robotic assembler machines may be repeatedly re-programmed with software to handle a variety of assembly tasks and parts, many conventional inspection and feed systems are not as versatile.
Numerous types of inspection devices have been designed to prevent misoriented, defective and otherwise non-conforming components from reaching assembly lines. At factories where components are manufactured, such inspection devices may be utilized to detect and help correct flaws resulting from manufacturing processes. At factories where components are assembled, such inspection devices may be utilized to detect conditions that cause defects in assembled products. These automated inspection devices often replace human inspectors, especially for inspection tasks which are difficult, tedious or expensive for humans to perform.
To improve the effectiveness and reduce the cost of automated inspection devices, attempts have been made to provide machines with the ability to remotely discern geometrical characteristics of objects--in essence, to "see" objects without physically touching them. (The phrase "geometrical characteristics" refers to external physical properties of objects and includes parameters such as position, orientation and shape.) To this end, some automated inspection systems have used optical equipment to sense changes in light reflected from objects. Although some optical systems have been successful, such systems often require extensive capabilities fo signal processing and computation.
Automated inspection systems of a somewhat different nature have been provided to recognize geometric characteristics of objects through transmission and reception of reflected waves in acoustic or electromagnetic fields that interact with inspected objects; the diffraction or scattering patterns of waves in the fields provide distinct "signatures" of the objects. In this regard, attention is drawn to U.S. Pat. Nos. 4,095,475; 4,200,921; 4,287,769; 4,479,241; 4,557,386 and 4,576,286 to Bruce S. Buckley. Such inspection systems, as compared to optical systems, usually require less capacity for signal processing and computation.
In one type of inspection system utilizing acoustic fields, measurement is made of time delays between sending and first receiving reflected sonic pulses. To completely determine shapes by such techniques, a transmitter and receiver must scan inspected objects, or multiple transmitters and receivers must be provided so that sound waves are reflected from various facets of inspected objects. In either case, facets of objects further from transmitters and receivers provide longer time delays than facets closer to transmitters and receivers. Measurement difficulties arise in such systems, however, because facets closer to transmitters and receivers than a facet of interest can mask measurement of the latter facet because pulses reflected from it are not "first received."
In another type of acoustic inspection system, continuous acoustic waves with a broad range of controllably-varied frequencies are used to establish fields about inspected objects, and measurements are made of the frequencies of reflected waves. In this technique, known as "continuous wave frequency modulation", electrical signals corresponding to reflected acoustic waves are provided by transducers and electronically translated into frequency domain signals, usually by Fourier transform algorithms. In such systems, geometrical characteristics can be determined by frequency modulation techniques because facets of objects further from transmitters and receivers provide responses at higher frequencies than facets closer to transmitters and receivers.
In still another type of acoustic inspection system, single-frequency continuous acoustic waves, usually sinusoidal, are directed towards objects presented for inspection. This technique is known as "phase-monitoring" because early systems measured only the phase of reflected waves; however, present systems provide both phase and amplitude information for reflected waves. According to phase-monitoring techniques, reflected waves have the same frequency as transmitted waves but differ from transmitted waves in amplitude and phase. An advantage of phase-monitoring techniques is that highly accurate digital and analog filters can be used; such filters, because they operate at a single frequency, are more easily constructed than the broadband filters required in frequency modulation techniques. Single-frequency phase-monitoring inspection systems are taught, for example, in U.S. Pat. Nos. 4,200,921 and 4,287,769. In phase-monitoring systems, geometrical characteristics of inspected objects can be determined because facets of objects further from receivers provide reflected waves having greater phase differences relative to emitted waves than facets closer to receivers. Because the acoustic signals are not pulsed in phase-monitoring systems, the systems avoid problems arising from close facets masking further facets and from multiple reflections as are encountered in other types of inspections systems utilizing acoustic fields
In acoustic systems using phase-monitoring techniques, it is typical to provide one or more arrays of transmitting transducers to establish acoustic fields and, likewise, to provide one or more arrays of receiver transducers to detect distortions, or scattering, caused by inspected objects in the fields. Phase and amplitude information in th systems is compared, usually by a programmed computer, to corresponding information for objects that are known to be non-defective. Computer control is used to orchestrate the data gathering processes and to analyze amplitude and phase information. In some instances, pattern-recognition algorithms can be employed to reduce field information to a scalar quantity that indicates the extent of similarity of inspected objects to objects of known geometrical characteristics.
With respect to inspection systems that use acoustic techniques to detect geometrical characteristics of objects, it is known that temperature variations along transmission paths can affect acoustic measurements. Such temperature variations can occur in the atmosphere within the inspection system and in the acoustic transducers themselves. Thermal effects on measurements arise because the speed of sound generally increases with the temperature of materials defining the transmission path. For example, air temperature variations along an acoustic transmission path can cause substantial uncontrolled "drift" in acoustic signal measurements; normally, drift in wave properties must be minimized in acoustic systems to maintain inspection integrity and accuracy.
It is also known that automated acoustic inspection systems can employ statistical methods to process electrical signals derived from the acoustic waves. In such systems, inspected objects are deemed non-defective if waves reflected from the objects have characteristics that fall within predetermined statistical ranges, usually expressed in terms of standard deviations or variances. Also, acoustic inspection systems that employ statistical methods are especially adaptable for "learning" processes. Learning processes normally depend upon "teaching" a system by initially inspecting objects that are known to be acceptable and, then, deriving statistical measures for comparison purposes with objects of unknown configurations that are presented for inspection. The effectiveness of such learning processes depends upon the extent to which signals associated with acceptable objects have stable statistical deviations; if reference signals drift substantially, statistical ranges may change, measurements may become inexact, and frequent "relearning" may be necessary.
In the environment of a typical factory, temperature variations of several degrees can occur rapidly enough to substantially affect acoustic inspection systems. For example, factory temperatures can be rapidly changed by two or three degrees by opening exterior doors or windows, by operating HVAC (heating, ventilating and air-conditioning) equipment, or by operating machinery in the factory. To avoid signal drift in acoustic inspection equipment in such environments, the prior art teaches that temperatures along acoustic transmission paths and at transducers in acoustic inspection equipment should be held constant independent of ambient conditions. Typically, temperature control within inspection equipment is accomplished by heaters and thermostatic controls. Regardless of the accuracy of such controls, however, unexpected and undesired wavelength variations of substantial magnitude have been observed; the variations have been detected by repeated inspections of stationary objects, and have been found to be more severe in factory environments than in laboratories.
A method to compensate for the effects of temperature on acoustic signals in phase-monitoring systems is suggested in U.S. Pat. No. 4,287,769. As explained in the patent, acoustic phase-monitoring systems use acoustic wavelength as the standard unit for detecting dimensions of inspected objects, and accurate measurements become problematical when acoustic wavelengths vary due to changes in temperature. The patent also points out that acoustic wavelengths will change with humidity and with velocity of a transmission medium. According to the patent, compensation for changes in transmission medium properties can be made by open-loop controls. In one open-loop control technique, for example, temperature changes in the transmission medium are measured and, based upon the measurements, an algorithm is employed to alter the frequency of emitted acoustic waves to maintain constant acoustic wavelength. The suggested algorithm, which is applicable to many gaseous mediums, is: ##EQU1## where f is the emitted frequency, .lambda. is the wavelength to be maintained constant, .gamma. is the ratio of specific heats for the medium, R is the universal gas constant, and T is temperature (absolute) of the transmission medium. In situations where frequency adjustments based upon the preceding algorithm did not completely compensate for temperature-related changes in the properties of the transmission medium, the lack of compensation was believed to be caused, for example, by errors in the assumed values of .gamma. and R, or errors in measuring temperature T, or by computation errors.
Regardless of the cause, however, uncontrolled wavelength changes in acoustic inspection systems of the phase-monitoring type can result in inspection errors. Such errors are usually categorized either as false acceptances or false rejections. A "false acceptance" is usually defined as an error arising from accepting objects which are defective; a "false rejection" error is usually defined as an error arising from rejecting objects which are not defective. False acceptance errors can severely affect automated assembly operations and thus can be quite costly and time consuming. False rejection errors, although often having less serious immediate consequences than false acceptance errors, nevertheless can cause problems if rejection rates for acceptable objects are high. Accordingly, efforts should be made to avoid both type of errors.